1. Field of the Invention
The present invention relates to a structure of an electrophotographic image forming apparatus with a photosensitive device integrated with an exposure source.
2. Description of the Related Art
In an electrophotographic process, a photosensitive member is uniformly charged and then exposed to light with a desired pattern based on image information so as to form a charge density distribution (latent image) on a surface of the photosensitive member. After that, the charge density distribution thus formed is developed with toner, to thereby obtain a visible image.
As a product to which the electrophotographic process is applied, a laser printer and an LED printer are widely used.
In the laser printer, a semiconductor laser is used as an exposure source, and a laser beam of the semiconductor laser is reflected by a rotating polygon mirror to thereby perform scanning on the photosensitive member.
In this case, in the following description, a main scanning direction of the rotary drum-shaped photosensitive member indicates a longitudinal direction of the drum (drum generatrix direction). Further, a sub-scanning direction of the rotary drum-shaped photosensitive member indicates a circumferential direction of the drum.
In the LED printer, there is employed a method in which the required number of light emitting diode (LED) pixels are arranged in a laser scanning direction (main scanning direction) of the laser printer, thereby forming an image on the surface of the photosensitive member by use of an imaging device.
The LED printer is characterized in that image positioning accuracy is enhanced because main scanning involved in the laser printer is not performed in the LED printer.
However, in both the laser printer and the LED printer, accuracy of sub-scanning is determined depending on a relative position and a relative speed between the photosensitive drum and the exposure source. Accordingly, unevenness in pitch is generated in a sub-scanning direction due to, for example, vibration of the exposure source, decentering of the photosensitive drum, and fluctuation in rotational speed.
In order to enhance the accuracy of the sub-scanning, it is possible to reduce a relative speed between the exposure source and the photosensitive member to zero. Specifically, it is possible that the exposure source and the photosensitive member are to be integrated with each other. As examples of the method of obtaining the integrated structure, the following methods have been employed.
(1) An example of a flat-plate photosensitive device in which a photoconductive layer is stacked on a light emitting device through an intermediate buffer layer
Japanese Patent Application Laid-Open No. H05-221018 discloses introduction of the intermediate buffer layer, as a method of stacking an a-Si photoconductive layer (amorphous silicon photoconductive layer) with high hardness on a thin-film electroluminescence (EL) layer.
(2) An example of a flat-plate photosensitive device in which an a-Si photoconductive layer is stacked on a light emitting array layer through an insulating layer.
Japanese Patent Application Laid-Open No. H06-095456 discloses a top emission structure of an inorganic LED in which a pixel thin-film-transistor (TFT) matrix is formed on a glass substrate.
(3) An example of a photosensitive drum in which a photoconductive layer is stacked on an electroluminescence (EL) device including a pixel TFT
Japanese Patent Application Laid-Open No. 2001-018441 discloses a device transfer process as a method of forming the EL device including a TFT layer on a cylindrical substrate.
In this case, the rotary drum-shaped photosensitive member, in which the exposure source and the photosensitive member are integrated with each other, that is, the drum integrated with the exposure source, in which pixels are formed on the photosensitive member so as to eliminate the factor of deviation in positional accuracy of an image not only in the main scanning direction but also in the sub-scanning direction, is hereinafter referred to as a digital photosensitive drum.
It is appropriate for a direction of technical development to employ the method of using the digital photosensitive drum in view of the technical transition from point scanning with a laser beam to an LED array in which the main scanning direction is fixed, and further, from the LED array to a pixel matrix system in which the sub-scanning direction is also fixed.
However, in a laser scanner for performing laser scanning and in the LED array for an LED system, the exposure source is spatially fixed and an image of the light source is formed on a spatially predetermined position. On the other hand, in the digital photosensitive drum, scanning lines are rotated with the drum. For this reason, there arises a problem to be solved for image formation. In other words, in a case of image formation using the digital photosensitive drum, as a first problem, it is necessary to employ a method of determining a scanning line to be exposed to light from an outside from the necessity that an exposure process is performed between a charging process and a development process for the image formation. As a second problem, in an in-line color image forming apparatus, in a case of correction control for matching positions of colors in a sub-scanning direction, it is necessary to provide a unit for determining a scanning line to be used after the correction, to each digital photosensitive drum for each color.
In the laser scanner for performing laser scanning and in the LED array for the LED system, the exposure source is spatially fixed and the image of the light source is formed on a spatially predetermined position.
However, in the digital photosensitive drum, the exposure source is rotated with the drum. Accordingly, in the case of image formation using the digital photosensitive drum, it is necessary to determine which exposure source performs an exposure process from the outside.
For the image formation, it is necessary to perform the exposure process between the charging process and the development process, and to perform exposure at a timing between the charging process and the development process. Further, in the in-line color image forming apparatus, it is necessary to determine an exposure timing for each drum so as to match the positions of the colors in the sub-scanning direction.
A conventional system is disadvantageous in the above-mentioned problems. In other words, in structures disclosed in Japanese Patent Application Laid-Open Nos. H05-221018 and H06-095456, a flat-plate device having the exposure source and the photosensitive member which are integrated with each other is used. Accordingly, in the first place, the structures are unsuitable for the electrophotographic image forming apparatus which is required to perform a continuous printing operation.
Further, in the structure of the digital photosensitive drum disclosed in Japanese Patent Application Laid-Open No. 2001-018441, a self-luminous device is wound around the drum substrate, so a seam is formed in the circumferential direction of the drum. For this reason, there is a description that a rotation start position (home position) of the drum is detected, and then, the image formation is performed after the elapse of predetermined time. However, with the structure, an exposing position (selection of scanning line) depends on time. Accordingly, when an image forming speed (rotational speed of drum) is changed, an error is generated in the exposure timing.